1. Field
This disclosure generally relates to construction of airfoils such as aircraft wings, and deals more particularly with attachment of wing ribs to tubular wing spars having variable cross sectional geometries.
2. Background
High altitude, long endurance (HALE) solar powered aircraft have been designed that will remain aloft continuously for extended periods. During the day, on-board solar arrays generate electricity that is used to power electric propulsion motors, and an electrolyzer that generates hydrogen and oxygen which is stored in on-board tanks. At night, the electric propulsion motors may be powered by a fuel cell that uses a combination of the hydrogen and oxygen stored in the tanks to generate electricity. Thus, the tanks are cyclically pressurized as the hydrogen and oxygen in the storage tanks are repeatedly drawn down and then replenished.
In order to reduce weight and achieve performance requirements, the aircraft may be fabricated from lightweight structures that are integrated with the aircraft's energy storage system. For example, tubular wing spars may function as storage tanks to hold the gaseous hydrogen and oxygen used by the fuel cell to produce electricity. Optimum spar weight may be achieved using an unsymmetric laminate construction which, because of interlaminar stresses, warps the spar into an ovoidal cross sectional shape. As pressurized gas is introduced into the spar, the spar gradually deforms and its cross sectional shape changes from an ovoid to nearly circular. As gas is drawn from the spar, its cross sectional shape springs-back to an ovoid. Spar deformation caused by this cyclical gas pressurization may result in the transmission of stresses to wing ribs to which the spar may be fixedly attached. These transmitted stresses may alter the shape of the rib and thus the overall shape of the airfoil, thereby affecting the aerodynamic performance of the wing.
Accordingly, there is a need for a joint for attaching wing ribs to a tubular spar that substantially isolates the ribs from stresses caused by changes in the cross sectional shape of the spar due to cyclical pressurization.